If you’re just looking for something to use to have fun with and never plan on racing with it then there’s a lot of options. Any older 125cc dirt bike engine would work with some slight modifications or you can use an older ICC engine which are engines built specifically for shifter karts.
The 99 honda cr125 will probably have the most parts / support available with a close second would be TM KZ icc depending on where you live.
Here’s a link to how to build a stock 99 honda cr125 for kart racing:
For reference here’s a link to an older ICC package on ebay:
Honestly your best bet just starting out is to find a local shop or track and pay them a visit to see if they can point you in the right direction.
November 15-19, 2017
Las Vegas, Nevada
SKUSA SuperNationals 21
2018 Events TBA